As is known, modulators are used in the field o analog communication systems, e.g. in amplitude modulated signal (AM) radio receivers.
Modulators usually comprise frequency multiplier circuits which are input with a pair of signals and output a proportional signal to the product of the two input signals. In particular, to a first input there is applied an oscillating signal of constant amplitude, commonly referred to as the carrier, which is generated locally by an oscillator incorporated to the receiver, whilst to a second input, there is applied a modulating analog signal from the radio receiver antenna.
The modulator is essentially a frequency translation circuit and its function in radio receivers is basically that of generating the amplitude modulated signals. Further, to perform the modulation with suppression of the carrier, use is made of so-called balanced modulators (mixers).
A typical balanced modulator comprises an output stage consisting of a pair of differential cells connected in parallel to each other and having the carrier signal applied to their inputs. Said pair is powered from the respective collectors of a further differential cell having the modulating signal from the antenna applied to its inputs. As a rule, the modulating signal from the antenna is within a frequency range from 560 to 1,600 kHz.
The modulator output is connected, via a coupling circuit, to a ceramic filter for selecting a modulated signal at a so-called fixed intermediate frequency (IF), at 455 kHz, of all those appearing at the modulator output.
To implement that coupling circuit, as well as adapting the output impedance of the modulator to the input impedance of the ceramic filter, the prior art proposes the use of an inductive type of impedance transformer.
The impedance transformer has the terminals of its primary winding respectively connected to a power supply pole V.sub.S and one of the modulator output terminals. In particular, said output terminal is a collector of the differential pair forming the modulator output stage, whilst the other collector is connected to the power supply pole V.sub.s.
Also connected between said terminals of the primary winding is a capacitor, and the secondary winding is connected between ground and the afoeesaid ceramic filter.
That prior coupling circuit, while performing substantially as expected, has drawbacks originating from the high manufacturing cost of the transformer and the need to calibrate and adapt it to the intermediate frequency IF.